The in-mold coating process is described in U.S. Pat. No. 4,081,578, while the auxiliary forced-velocity leveling unit is described in U.S. Pat. No. 4,076,780.
The leveling unit is an arrangement of pistons around the periphery of the stationary mold half that act in opposition to the closing mold half, and serve to maintain the opposing mold faces parallel during closure. The total push-back pressure is adjustable, and is set to some fraction of the pressing tonnage. The individual pressure in a given piston will vary automatically with the requirements of maintaining parallelism; the pressure sum of all pistons, however, equals the pre-set value.
Ideally, the unit works as follows. During the initial SMC (sheet molding compound to make an FRP, glass fiber reinforced plastic part) molding, parallelism is essentially maintained, insuring more uniform part thickness, according to the mold's dimensions, with a minimum of the customary part to part and area to area thickness variability due to random press rocking during closure. After the SMC is cured, the main ram is turned off, while the leveling unit opens the press a pre-set amount, usually 25 to 100 mils. At this point, the in-mold coating is injected, ram pressure is turned on, and the press closes a second time with near parallelism, insuring that the coating spreads evenly and uniformly over the SMC surface. The IMC injector is located at an edge of the mold, and the injection port is an integral part of the mold.
In practice, the leveling unit minimizes non-parallelism but does not totally eliminate it. Several factors affect the degree of departure from non-parallelism, the main one being the initial location of the SMC charge placement. For example, if a charge is placed towards one end of the mold cavity, the leveling unit sometimes cannot totally overcome the resulting torque, and the molded part will tend to be thick at the charge location, thinning down towards the opposite side. With the leveling unit off, the degree of this slope would be much higher. Even with a centrally located charge, while the degree of non-parallelism is fairly low, the orientation of the slope will vary from part to part because of residual, random press rocking during closure.
According to experience, the minimum amount of in-mold coating needed for full part coverage is partly a function of the degree of non-parallelism of the base SMC part, and specifically the slope orientation with respect to the IMC injection port. The ideal orientation is the case where the part is thickest near the injection port, and thins down diagonally across the part away from this location. It is speculated that during the IMC cycle, with this orientation, the base part and the closing mold will come in contact at the thick section first (where the IMC puddle is located). The final few mils of closure will then sweep the coating across the base part in a scissors-like action as the closing halves are forces to conform to the skewed base part. Conversely, had the slope orientation been such that the thick section of the base part was located away from the IMC injection port, the coating would have to flow towards the fulcrum of the scissors action or, therefore, into a converging cavity. Experience shows that this situation requires a maximum of in-mold coating.
A manufacturer, for obvious reasons, does not wish to apply more coating than is needed for complete coverage. This minimum is usually determined by trial and error, a safety margin is added, and the injector is set to delivery this amount of coating. The difficulty is that the minimum coating required is a function of the base part's slope orientation which, even with leveling, has enough variability so that an unfavorable slope orientation would result in partial coverage. This in turn results in either expensive repair work or a scrap part.
Rather than increasing the "safety margin" as described above, it has become standard practice to place the SMC charge close to the IMC injection port, thus guaranteeing a favorable base part slope. The price paid for this approach, however, is three-fold. First, the purpose of the leveling unit is defeated, and the manufacturer reproducibly produces parts that are skewed in thickness. Secondly, the molder's charge placement options are eliminated. It is well known that with a complicated mold cavity that includes asymmetry, substructure, etc., many problems such as poorly placed knit lines, ripples, poor flow, etc. can be minimized by the proper choice of SMC charge placement. Thirdly, by restricting charge placement to an area close to the injection port, the flow path of the SMC may be excessively long, leading to glass orientation, resin rich areas, fracture, etc. at the point of furthest flow.
A second approach that has not been used extensively in practice is to shim the mold favorably during the SMC molding cycle, but removing the shims during the IMC molding cycle. In the normal course of molding, when fully closed the two halves of a mold are separated by metal blocks, called stops, placed around the edges of the platens. The thickness of these stops defines the minimum thickness that a part can be molded to. By placing a shim on a stop close to the IMC injection port, the molder can insure a favorably sloped part more or less independent of charge placement. This would give the desired "scissors action." While in principle removing the restriction on charge placement, this method still results in a skewed base part.
An object of the present invention is to avoid the difficulties alluded to above and to provide a method of making a FRP part from SMC wherein the outer surface of the SMC molding is at least essentially completely coated with a minimum amount of an IMC composition.